Skill Change Suggestions

So I finally have Balrog+ and I'm working on leveling him up now to put in my formation, but his Covert Down ability overlaps with my Paimon+'s ability. As I would naturally like to get the most out of every monster I can, I'm curious what options I should look to as far as a skill change. Should I change Balrog or Paimon's Skill? What skill should I consider making the switch to? Please note that I am also using Spite+ at the moment, so Decrease Attack would not help. Balrog will be replacing Djinn, and my DDD will very likely be replaced by Asura down the line. Is Thunder worth it? What other options exist, and what should I expect to pay for each?

Thanks for suggestions!

I put celestial lightning on my balrog ... I personally enjoy it, although a few people have told me I was foolish to do so ... :/